Network SecurityNetwork SecurityConfidentiality Using Confidentiality Using Symmetric EncryptionSymmetric Encryption

Chapter 7

Symmetric Key Cryptography

EncryptionEncryption

““The quick The quick brown fox brown fox jumps over jumps over the lazy the lazy dog”dog”

““AxCv;5bmEseTfid3)AxCv;5bmEseTfid3)fGsmWe#4^,sdgfMwifGsmWe#4^,sdgfMwir3:dkJeTsY8R\s@!r3:dkJeTsY8R\s@!q3%”q3%”

““The quick The quick brown fox brown fox jumps over jumps over the lazy the lazy dog”dog”

DecryptionDecryption

Plain-text input Plain-text outputCipher-text

Same key(shared secret)

Confidentiality using Symmetric Encryption

• Traditionally symmetric encryption is used to provide message confidentiality

• Consider a typical scenario

– Workstations on LANs access other workstations & servers on LAN

– LANs are interconnected using switches/routers

– With external lines or radio/satellite links

Points of Vulnerability

Confidentiality using Symmetric Encryption

• Consider attacks and placement in this scenario– snooping from another workstation– use dial-in to a LAN or a server to snoop– use external router link to enter & snoop– monitor and/or modify traffic on external

links

Confidentiality using Symmetric Encryption

• Have two major placement alternatives

– Link Encryption

– End-to-End Encryption

Location of Encryption DeviceLink Encryption

• Encryption devices are placed at each end of the link

• Encryption occurs independently on every link

• All the communication is made secure

• A lot of encryption devices are required

• Decrypt each packet at every switch

• High level of security

Link Encryption Implications

• All paths must use link encryption

• Each pair of node must share a unique key

– Large number of keys should be provided

End-to-End Encryption

• Source encrypts and the Receiver decrypts

• Payload encrypted

• Header in the clear

• Only destination and reciever share the key

• High Security: Both link and end-to-end encryptions are needed

Encryption Across a Packet Switching Network

Traffic Analysis• When using end-to-end encryption must

leave headers in clear

– So network can correctly route information

• Although content is protected, traffic flow patterns are not

• Ideally want both at once

– End-to-End protects data contents over entire path and provides authentication

– Link protects traffic flows from monitoring

Placement of Encryption

• Can place encryption function at various layers in OSI Reference Model

– Link encryption occurs at layers 1 or 2

– End-to-End can occur at layers 3, 4, 6, 7

– As move higher, less information is encrypted but it is more secure and more complex with more entities and keys

Encryption coverage implications of store and forward communications

Traffic Analysis

• Monitoring of communications flows between parties

– Useful both in military & commercial spheres

• Link encryption obscures header details

– But overall traffic volumes in networks and at end-points is still visible

• Traffic padding can further obscure flows

– But at cost of continuous traffic

Traffic Padding Encryption Device

Required Key Protection

CONFIDENTIALITYCONFIDENTIALITY

AVAILABILITYAVAILABILITY

INTEGRITYINTEGRITY

AUTHENTICATIONAUTHENTICATION

Key Storage• In Files

– Using access control of operating system

• In Crypto Tokens– Smart card, USB crypto token– Supports complete key life-cycle on token

• Generation – storage – use – destruction – provide means to ensure that there is no

way to get a key out

• Key Backup (also known as key escrow)

Number of keys required to support Arbitrary

connections

Use of a Key Hierarchy

Key Renewal• Keys should be renewed

• More available cipher texts may facilitate certain attacks

• How often depends on the crypto algorithm

– Can depend on the amount of encrypted data

– May depend on time (exhaustive key search requires time)

• Regular key renewal can reduce damage in case of (unnoticed) key compromise

• Protocols like SSL/TLS include features for (secret) key renewal

Key Life-Cycle

Time

Key Generation

Key Destruction

Key Storage and

Usage

Keys must be protected

Requires a secure random source!

Unrecoverable deletion

Key Distribution

• Means of Exchanging Keys between two parties

• Keys are used for conventional encryption

• Frequent key exchanges are desirable

– Limiting the amount of data compromised

• Strength of cryptographic system rests with Key Distribution Mechanism

Key Distribution

• Symmetric schemes require both parties to share a common secret key

• Issue is how to securely distribute this key

• Often a secure system failure due to a break in the key distribution scheme

Key Distribution

• Two parties A and B can have various key distribution alternatives:

1. A can select key and physically deliver to B2. third party can select & deliver key to A & B3. if A & B have communicated previously can

use previous key to encrypt a new key4. if A & B have secure communications with a

third party C, C can relay key between A & B

Key Distribution Scenario

Key Distribution Scenario

1. A issues a request to the KDC for a session key– Nonce is also sent– Nonce includes identities of communicating

parties and a unique value

2. KDC sends a response encrypted with A’s secret key KA

– It includes one time session key KS

– Original request message, including the nonce– Message also includes KS and ID of A

encrypted with KB intended for B

Key Distribution Scenario

3. A stores KS and forwards information for B i.e., EKB

[KS||IDA]

4. B sends a nonce to A encrypted with KS

5. A responds by performing some function on nonce like incrementing

The last two steps assure B that the message it received was not a replay

Key Distribution Entities

• Key Distribution Center– Provides one time session key to valid

users for encryption• Front end Processor

– Carries out the end to end encryption– Obtains session key from the KDC on

behalf of its host

Key distribution for symmetric keys

• Key distribution for symmetric keys by a central server (KDC):- fixed number of distributions (for given

n)- However, need security protocol

Key Distribution Issues Hierarchical Key Control

• Not suitable that a single KDC is used for all the users

• Hierarchies of KDC’s required for large networks

• A single KDC may be responsible for a small number of users since it shares the master keys of all the entities attached to it

• If two entities in different domains want to communicate, local KDCs communicate through a global KDC

• Must trust each other

Session Key Lifetimes

• Session key lifetimes should be limited for greater security

• More frequently the session keys are exchanged, more secure they become

• For connection oriented protocols, it should be valid for the duration of connection

• For connectionless protocols key should be valid for a certain duration

Transparent Key Control

• Use of automatic key distribution on behalf of users, but must trust system

1. Host sends packet requesting connection

2. Front End buffers packet; asks KDC for session key

3. KDC distributes session key to both front ends

4. Buffered packet transmitted

Automatic Key Distribution for Connection-Oriented Protocol

KDC

HOSTHOSTFEP

FEP

FEP

Decentralized Key Control

• KDCs need to be trusted and protected• This can be avoided by the use of decentralized

key distribution• Decentralized approach requires that each node

be able to communicate in a secure manner • Session key may be established in following way

1. A issues a request to B for a session key and includes a nonce, N1.

2. B responds with a message that is encrypted using the shared secret key• Response includes session key, ID of B, the value

f(N1) and nonce N23. Using the new session key, A returns f(N2) to B

Decentralized Key Distribution

Controlling Key Usage• Different types of session keys e.g.,

– Data encrypting key: for general communication across network

– PIN-encrypting key: for PIN used in electronic funds

– File encrypting key: for encrypting files stored on a publicly accessible location

• Avoid using master key instead of session key since some unauthorized application may obtain the master key and exploit it

Key Distribution

• Session key– Data encrypted with a one-time session

key. At the conclusion of the session the key is destroyed

• Permanent key– Used between entities for the purpose of

distributing session keys

Summary

• Have considered:– use of symmetric encryption to protect

confidentiality– need for good key distribution– use of trusted third party KDC’s